import java.util.StringTokenizer;
import java.util.Stack;

public class MyDC
{
    // constant for addition symbol */
    private final char ADD = '+';
    // constant for subtraction symbol */
    private final char SUBTRACT = '-';
    // constant for multiplication symbol */
    private final char MULTIPLY = '*';
    // constant for division symbol */
    private final char DIVIDE = '/';
    // the stack */
    private Stack<Integer> stack;

    public MyDC() {
        stack = new Stack<Integer>();
    }

    public int evaluate (String expr)
    {
        int op1, op2, result = 0;
        String token;
        StringTokenizer tokenizer = new StringTokenizer (expr);

        while (tokenizer.hasMoreTokens())
        {
            token = tokenizer.nextToken();

            //如果是运算符，调用isOperator

            if (isOperator(token))
            {
                //从栈中弹出操作数

                //op2 =Integer.parseInt(stack.pop());
                op2 =stack.pop();
                //从栈中弹出操作数
                //op1 =Integer.parseInt(stack.pop());
                op1 =stack.pop();
                //根据运算符和两个操作数调用evalSingleOp计算result;
                result = evalSingleOp(token.charAt(0),op1,op2);
                //计算result入栈;
                stack.push(result);
            }
            else//如果是操作数
                //操作数入栈;
                stack.push(Integer.parseInt(token));
        }

        return result;
    }

    private boolean isOperator (String token)
    {
        return ( token.equals("+") || token.equals("-") ||
                token.equals("*") || token.equals("/") );
    }

    private int evalSingleOp (char operation, int op1, int op2)
    {
        int result = 0;

        switch (operation)
        {
            case ADD:
                result = op1 + op2;
                break;
            case SUBTRACT:
                result = op1 - op2;
                break;
            case MULTIPLY:
                result = op1 * op2;
                break;
            case DIVIDE:
                result = op1 / op2;
        }

        return result;
    }
}